Hot worked steel and tool made therewith

ABSTRACT

A hot worked matrix steel consisting of (in weight-%) 0.60-0.85 C, 0.05-0.5 Si, ≦0.5 (Si+Al), 0.1-2.0 Mn, 4.5-5.5 Cr, 1.5-2.6 Mo, ≦1.0 W, 1.5-2.6 (Mo+W/2), 0.42-0.65 V, ≦0.1 Nb, ≦0.1 Ti, ≦0.1 Zr, ≦2.0 Co, ≦2.0 Ni, ≦0.003 S, optionally, up to 30 ppm B, balance iron and unavoidable impurities. The steel after hardening and tempering at 520-600° C. (2×2 h) has a hardness of 57-63 HRC and an un-notched impact energy in the transverse direction of 20-100 J. The steel consists of tempered martensite. The steel contains 1.04 vol. % or less of primary precipitated vanadium carbides. The steel is void of primary carbides other than primary precipitated vanadium carbides.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of the U.S. National Phase patentapplication Ser. No. 10/514,939, filed Jan. 14, 2005, which is the U.S.National Phase of International Patent Application No. PCT/SE03/00940,filed Jun. 6, 2003, which claims priority to Swedish Patent ApplicationNos. 0201799-4 filed Jun. 13, 2002, and 0300200-3, filed Jan. 29, 2003.The entire contents of all of the aforementioned applications areincorporated herein by reference.

FIELD OF THE INVENTION

The invention concerns a cold work steel i.e. a steel intended to beused for working materials in the cold condition of the material.Punches and dies for cold forging and other cold-pressing tools,cold-extrusion tools and thread rolling dies, but also cutting tools,e.g. knives, such as sharing knives for cutting sheet, circular cutters,and the like are typical examples of the use of the steel. The inventionalso concerns the use of the steel for the manufacturing of cold worktools as well as tools made of the steel.

BACKGROUND OF THE INVENTION

It is the purpose of the invention to provide a cold work steel whichcan be used inter alia for the above applications and which thereforeshould have the following features:

-   -   Good ductility/toughness    -   Good hardenability allowing through hardening in connection with        conventional hardening in a vacuum furnace of products with        thicknesses up to at least 300 mm,    -   Adequate hardness, at least 60 HRC, after hardening and high        temperature tempering, which gives a high resistance against        plastic deformation and, at least as far as certain applications        are concerned, also an adequate wear resistance without        nitriding or surface coating with titanium carbide and/or        titanium nitride or the like by means of e.g. PVD- or        CVD-technique,    -   Good tempering resistance in order to allow nitriding or surface        coating with titanium carbide and/or titanium nitride or the        like by e.g. any of said techniques without reduction of the        hardness of the material, for applications which require        particularly good wear resistance of the tool.

Other important product features are:

-   -   Good dimension stability during heat treatment,    -   Long fatigue life,    -   Good grindability, machinability, spark machinability, and        polishability.

Specifically, the invention aims at providing a matrix steel which canbe employed for the above applications, i.e. a steel which isessentially void of primary carbides and which in use condition has amatrix consisting of tempered martensite.

DISCLOSURE OF THE INVENTION

The above mentioned purposes and features can be achieved by means of asteel which is characterised by what is stated in the appending patentclaims.

As far as the individual elements of the steel alloy and their mutualinteraction are concerned, the following applies.

The steel of the invention shall, as above mentioned, not contain anyprimary carbides or only an extremely low content of primary carbides,i.e. be essentially void of primary carbides, but nevertheless have awear resistance which is adequate for most applications. This can beachieved by an adequate hardness within the range 57-63 HRC, suitably60-62 HRC, in the hardened and high temperature tempered condition ofthe steel, at the same time as the steel shall have a very goodtoughness. In order to achieve this, the steel contains carbon andvanadium in well balanced amounts. Thus the steel contain at least0.60%, preferably at least 0.63%, and suitably at least 0.68% C. Furtherthe steel should contain at least 0.30%, preferably at least 0.35%, andsuitably at least 0.42% V. This makes it possible that the martensiticmatrix in the hardened and tempered condition of the steel will containsufficient amount of carbon in solid solution in order to give saidhardness to the matrix, and also that an adequate amount of secondarilyprecipitated, very small, hardness increasing vanadium carbides will beformed in the matrix of the steel. Moreover, very small, primaryprecipitated vanadium carbides exist in the steel, which contribute tothe prevention of grain growth during the heat treatment. Any othercarbides than vanadium carbides should not exist. In order to achievesaid conditions, the steel must not contain more than 0.85%, preferablymax. 0.80%, and suitably max. 0.78% C, while the vanadium content mayamount to max. 0.65%, preferably max. 0.60%, and suitably max. 0.55%.Nominally, the steel contains 0.72% C and 0.50% V. The content of carbonin solid solution in the hardened and high temperature temperedcondition of the steel nominally amounts to about 0.67%.

Silicon exists at least in a measurable amount as a residual elementfrom the manufacturing of the steel and is present in an amount fromtraces up to max. 1.5%. Silicon, however, impairs the toughness of thesteel and should therefore not exist in an amount exceeding 1.0%,preferably max. 0.5%. Normally, silicon exists in a minimum amount of atleast 0.05%. An effect of silicon is that it increases the carbonactivity in the steel and therefore contributes to affording the steel adesired hardness. Another positive effect of silicon is that it mayimprove the machinability of the steel. Therefore it may be advantagesthat the steel contains silicon in an amount of at least 0.1%. Nominallythe steel contains 0.2% silicon.

Aluminium to some extent may have the same or similar effect as siliconat least in a steel of the present type. Both can be used as oxidationagents in connection with the manufacturing of the steel. Both areferrite formers and may provide a dissolution hardening effect in thematrix of the steel. Silicon therefore may be partly replaced byaluminium up to an amount of max. 1.0%. Aluminium in the steel, however,makes it necessary that the steel is very well deoxidised and has a verylow content of nitrogen, because aluminium oxides and aluminium nitridesotherwise would form, which would reduce the ductility/toughness of thesteel considerably. Therefore, the steel should normally not containmore than max. 1.0% Al, preferably max. 0.3%. In a preferred embodiment,the steel contains max. 0.1% and most conveniently max. 0.03% Al.

Manganese, chromium and molybdenum shall exist in a steel in asufficient amount in order to give the steel an adequate hardenability.Manganese also has the function of binding the extremely low contents ofsulphur which may exist in the steel to form manganese sulphides.Manganese therefore, shall exist in an amount of 0.1-2.0%, preferably inan amount of 0.2-1.5%. Suitably, the steel contains at least 0.25% andmax. 1.0% manganese. A nominal manganese content is 0.50%.

Chromium shall exist in a minimum amount of 3.0%, preferably at least4.0% and suitably at least 4.5% in order to give the steel a desiredhardenability when the steel contains manganese and chromium in amountswhich are characteristic for the steel. Maximally, the steel may contain7.0%, preferably max. 6.0% and suitably max. 5.5% chromium.

Also molybdenum shall exist in an adequate amount in the steel in orderto afford, together with in the first place chromium, the steel adesired hardenability and also to give it a desired secondary hardening.Molybdenum in too high contents, however, causes precipitation of M₆Ccarbides, which preferably should not exist in the steel. With thisbackground, the steel therefore shall contain at least 1.5% and max.4.0% Mo. Preferably, the steel contains at least 1.8% and max. 3.2% Mo,suitably at least 2.1% and max. 2.6% Mo in order that the steel shallnot be caused to contain undesired M₆C carbides at the cost of and/or inaddition to the desired amount of MC carbides. Molybdenum in principalcompletely or partly may be replaced by tungsten for the achievement ofa desired hardenability, but this requires twice as much tungsten asmolybdenum which is a drawback. Also recirculation of scrap which isproduced in connection with the manufacturing of the steel is made moredifficult if the steel contains substantial contents of tungsten.Therefore, tungsten should not exist in an amount of more than max.1.0%, preferably max. 0.3%, suitably max. 0.1%. Most conveniently, thesteel should not contain any intentionally added amount of tungsten,which in the most preferred embodiment of the steel should not betolerated more than as an impurity in the form of a residual elementemanating from used raw materials for the manufacturing of the steel.

In addition to the said elements, the steel normally need not containany further, intentionally added alloy elements. Cobalt, for example, isan element which normally is not required for the achievement of thedesired features of the steel. However, cobalt may optionally be presentin an amount of max. 2.0%, preferably max. 0.7%, in order to furtherimprove the tempering resistance. Normally, however, the steel does notcontain any cobalt exceeding impurity level. Another element whichnormally need not exist in the steel, but which optionally may bepresent, is nickel, in order to improve the ductility of the steel. Attoo high contents of nickel, however, there is a risk of formation ofretained austenite. Therefore the nickel content must not exceed max.2.0%, preferably max. 1.0%, suitably max. 0.7%. If an effective contentof nickel is considered to be desired in the steel, the content e.g. mayamount to 0.30-0.70%, suitably to about 0.5%. In a preferred embodiment,when it is considered that the steel has a sufficientductility/toughness also without nickel, the steel, in relation to costreasons, should not contain nickel in amounts exceeding that content ofnickel which the steel unavoidably will contain in the form of animpurity from used raw materials, i.e. less than 0.30%.

Further, the steel in a manner per se, can optionally be alloyed withvery small contents of different elements in order to improve thefeatures of the steel in various respects, e.g. its hardenability, orfor facilitating the manufacturing of the steel. For example, the steelmay optionally be alloyed with boron in contents up to about 30 ppm inorder to improve the hot ductility of the steel.

Other elements, on the other hand, are explicitly undesired. Thus, thesteel does not contain any other strong carbide formers than vanadium.Niobium, titanium, and zirconium, for example, are explicitly undesired.Their carbides are more stabile than vanadium carbide and require highertemperature than vanadium carbide in order to be dissolved at thehardening operation. While vanadium carbides begin to be dissolved at1000° C. and are in effect completely dissolved at 1100° C., niobiumcarbides do not start to be dissolved until at about 1050° C. Titaniumcarbides and zirconium carbides are even more stabile and do not startto be dissolved until temperatures above 1200° C. are reached and arenot completely dissolved until in the molten condition of the steel.Strong carbide and nitride formers other than vanadium, particularlytitanium, zirconium, and niobium, therefore must not exist in amountsabove 0.1%, preferably max. 0.03%, suitably max. 0.010%. Mostconveniently, the steel does not contain more than max. 0.005% of eachof said elements. Also the contents of phosphorus, sulphur, nitrogen andoxygen are kept at a very low level in the steel in order to maximisethe ductility and toughness of the steel. Thus, phosphorus may exist asan unavoidable impurity in a maximum amount of 0.035%, preferably max.0.015%, suitably max. 0.010%. Oxygen may exist in a maximal amount of0.0020% (20 ppm), preferably max. 0.0015% (15 ppm), suitably max.0.0010% (10 ppm). Nitrogen may exist in an amount of max. 0.030%,preferably max. 0.015%, suitably max. 0.010%.

If the steel is not sulphurised in order to improve the machinability ofthe steel, the steel contains max. 0.03% sulphur, preferably max. 0.010%S, suitably max. 0.003% (30 ppm) sulphur. However, one may conceive toimprove the machinability of the steel by intentional addition ofsulphur in an amount above 0.03%, preferably above 0.10% up to max.0.30% sulphur. If the steel is sulphurised, it may in a manner known perse also contain 5-75 ppm Ca and 50-100 ppm oxygen, preferably 5-50 ppmCa and 60-90 ppm oxygen.

During the manufacturing of the steel, there are produced ingots orblanks having a mass exceeding 100 kg, preferably up to 10 tons andthicknesses exceeding about 200 mm, preferably up to at least 300 or 350mm. Preferably, conventional melt metallurgical manufacturing isemployed via ingot casting, suitably bottom casting. Also continuouscasting may be employed, provided it is followed by recasting to desireddimensions according to above, e.g. by ESR remelting. Powder metallurgymanufacturing or spray forming are unnecessarily expensive processes anddo not give any advantages which motivate the cost. The produced ingotsare hot worked to desired dimensions, when also the cast structure isbroken down.

The structure of the hot worked material can be normalised in differentways by heat treatment in order to optimise the homogeneity of thematerial, e.g. by homogenisation treatment at high temperature, suitablyat 1200-1300° C. The steel is normally delivered by the steelmanufacturer to the customer in the soft annealed condition of thesteel; hardness about 200-230 HB, normally 210-220 HB. The tools arenormally manufactured by machining operations in the soft annealedcondition of the steel, but it is also conceivable per se to manufacturethe tools by conventional machining operations or by spark machining inthe hardened and tempered condition of the steel.

The heat treatment of the manufactured tools is normally carried out bythe customer, preferably in a vacuum furnace, by hardening from atemperature between 950-1100° C., suitably at 1020-1050° C., forcomplete dissolution of existing carbides, for a period of time between15 min to 2 h, preferably for 15-60 min, followed by cooling to 20-70°C., and high temperature tempering at 500-600° C., suitably at 520-560°C.

In the soft annealed condition of the steel, the steel has a ferriticmatrix containing evenly distributed, small carbides, which may be ofdifferent kind. In the hardened and not tempered condition, the steelhas a matrix consisting of untempered martensite. In terms ofcalculation by known theoretical calculations, the steel at equilibriumcontains about 0.6 vol-% MC carbides. At high temperature tempering, anadditional precipitation of MC carbides is obtained, which affords thesteel its intended hardness. These carbides have a sub microscopic size.The amount of carbides is therefore impossible to state by conventionalmicroscopic studies. If the temperature is increased too much, the MCcarbides are caused to be more coarse and become instable, which insteadcauses rapidly growing chromium carbides to be established, which is notdesired. For these reasons, it is important that the tempering isperformed at the above mentioned temperatures and holding times as faras the alloy composition of the steel of the invention is concerned.

Further features and aspects of the invention will be apparent from thepatent claims and from the following description of performedexperiments and from the subsequent discussion.

BRIEF DESCRIPTION OF DRAWINGS

In the following description of performed experiments, reference will bemade to the accompanying drawings, in which

FIG. 1-FIG. 5 concern investigations of steels manufactured at alaboratory scale, where FIG. 1 is a chart illustrating the influence ofthe tempering temperature on the examined steels,

FIG. 2 is a chart illustrating the hardenability of the examined steels,

FIG. 3 is a chart illustrating the ductility in terms of impacttoughness of examined materials versus the hardness of samples hardenedin a vacuum furnace at different cooling times,

FIG. 4 is a bar chart showing the ductility and the hardness of examinedsteel after a specific heat treatment, and

FIG. 5 is a chart illustrating the hot ductility of examined steels inthe cast and forged condition, respectively, of the steel, and

FIG. 6 and FIG. 7 concern examinations of steels manufactured at aproduction scale, where FIG. 6 illustrates the ductility of samples ofexamined steels, taken in some different positions in manufactured bars,and

FIG. 7 shows the microstructure of a steel according to the inventionafter heat treatment.

DESCRIPTION OF PERFORMED EXPERIMENTS Experiments at a Laboratory ScaleMaterials

Four steel alloys were manufactured in the form of laboratory ingotshaving a mass of 50 kg. The chemical compositions are given in Table 1.The sulphur content could not be maintained at a desirably low levelbecause of the limitations of the manufacturing technique. The contentof oxygen and of other impurities than those which are given in thetable have not been analysed. The following process sequence wasapplied: homogenisation treatment for 10 h at 1270° C./air, forging to60×60 mm, regeneration treatment 1050° C./2 h/air, and soft annealing850° C./2 h, cooling 10° C./h to 600° C., then free in air.

TABLE 1 Chemical composition in weight-% of materials manufactured at alaboratory scale Ti Nb N Steel C Si Mn P S Cr Mo V ppm ppm O ppm Balance1 0.68 0.87 0.65 0.005 0.006 2.82 2.34 0.52 33 <10 n.a. 14 Fe + otherimpurities 2 0.68 0.19 0.39 0.004 0.006 4.93 2.37 0.37 29 <10 n.a. 28Fe + other impurities 3 0.71 0.90 0.49 0.004 0.006 5.09 2.36 0.56 39 <10n.a. 19 Fe + other impurities 4 0.63 1.38 0.35 0.007 0.006 4.25 2.871.81 42 <10 n.a. 18 Fe + other impurities n.a. = not analysed

TABLE 8 Chemical composition in weight-% (S, B and O in ppm). balance Feand impurities of materials manufactured at a production scale Steel CSi Mn P S Cr Ni Mo W Co V Ti Nb Cu Al N B O 10 0.71 0.19 0.49 .009 64.96 0.07 2.28 .003 .010 0.50 .0016 .001 0.52 .017 .011 10 7 11 0.710.19 0.49 .009 8 4.98 0.07 2.30 .003 .011 0.50 .0015 .001 0.52 .015 .01110 5 12 0.74 0.99 0.76 .007 10 2.55 0.06 2.09 .01 .01 0.50 .003 .01 .07.037 .007 30 8

The above materials were examined with reference to hardness after softannealing, micro-structure after different heat treatments, hardnessafter hardening and tempering, hardenability, impact toughness, wearresistance, and hot ductility. These investigations are reported in thefollowing. Moreover, theoretical equilibrium calculations were carriedout by the Thermo-Calc method with reference to the content of dissolvedcarbon and carbide fraction at the indicated austenitising temperaturefor the steels having the aimed compositions according to table 2.

TABLE 2 Chemical composition, weight-%, of Thermo-Calc-studied alloysSteel C Si Mn P S Cr Mo V 5 0.72 1.00 0.75 0.02 0.005 2.60 2.25 0.50 60.71 0.20 0.50 0.02 0.005 5.00 2.30 0.55 7 0.74 1.00 0.50 0.02 0.0055.00 2.30 0.55 8 0.65 1.50 0.40 0.02 0.005 4.20 2.80 1.80

The content of dissolved carbon at the austenitising temperature, TA,and vol-% MC at

TA are stated in table 3 below.

TABLE 3 T_(A) (° C.) % C vid T_(A) vol-% MC vid T_(A) 5 1050/30 min 0.631.01 6 1050/30 min 0.65 0.72 7 1050/30 min 0.64 1.04 8 1150/10 min 0.382.87

Soft Annealed Hardness

The soft annealed hardness, Brinell hardness (HB), of the examinedalloys 1-4 is given in table 4.

TABLE 4 Soft annealed hardness Steel Hardness (HB) 1 218 2 208 3 217 4222

Micro-Structure

The micro-structure was examined in the soft annealed condition afterheat treatment to 60-61 HRC. These studies evidenced that themicro-structure in the hardened and tempered condition consisted oftempered martensite. Primary carbides occurred only in steel 4. Thesecarbides were of type MC. Any titanium carbides, -nitrides and/or-carbonitrides were not detected in any alloy.

Hardening and Tempering

The steels 1-3 were austenitised at 1050° C./30 min and steel 4 wasaustenitised at 1150° C./10 min, air cooled to ambient temperature andannealed twice at different tempering temperatures, each time for 2hours. The influence of the tempering temperature on the hardness isshown in FIG. 1. This figure indicates that the steels 2 and 3 have apotentiality to attain a desired hardness after high temperaturetempering at 500-600° C., preferably at 520-560° C., suitably 520-540°C. An optimum for maximal hardness is achieved by tempering at atemperature of about 525° C. as far as the steels 2 and 3 are concerned.This is particularly important for matrix steels, which requirenitriding or surface coating at a temperature in the order of 500° C. orhigher for the achievement of a wear resistance which is required forcertain tool applications. At these temperatures, it is thus achieved apronounced secondary hardening due to the precipitation of MC-carbides.As is apparent from the chart in FIG. 1, a hardness exceeding 60 HRC isguaranteed by tempering even up to about 580° C., which is advantageous,because it makes it possible to perform the surface coating within arather wide temperature range without causing the hardness of the toolto be too low. If a higher hardness is aimed at, more carbone and morecarbide forming element must be added to the alloy. This, however, wouldcause a risk for the formation of primary carbides, which can not bedissolved by annealing. This is exemplified by steel 4, which requires avery high austenitising temperature, which causes a number of drawbacks;requirement of an unconventional hardening technique applied by the toolmaker, hardening tensions, dimension changes, and risk of fissures.Hardenability

A comparison of the hardenability of the examined alloys 1-4, employingplotted data from CCT-diagrams, is shown in FIG. 2. As is shown by thediagram, steel No. 2 has the best hardenability, but also steel No. 3has better conditions for the formation of martensite when the steel isslowly cooled from the austenitising temperature in comparison withsteel No. 1 and definitely in comparison with steel No. 4.

Ductility

The ductility in terms of absorbed impact energy for un-notched testrods at 20° C., hardened in a vacuum furnace at different cooling times,and tempered to different hardnesses, is shown in FIG. 3. The besttoughness, when the hardness exceeded 60 HRC was achieved for steel No.2, and this effect was even more pronounced when the hardness exceeded61 HRC. In order further to analyse the toughness conditions at the saidhardnesses, the steels 1-4 were also compared in a bar chart, FIG. 4. Inthis case, the steels 1-4 were cooled from the above mentionedaustenitising temperature during 706 seconds from 800° C. to 500° C.,and, after continued cooling to room temperature, the steels weretempered at 525-540° C./2×2 h. FIG. 4 shows that the best toughness,when the hardnesses were comparable, was achieved with steel 2.

Hot Ductility

The hot ductility is an important parameter for, among other things, theproduction economy of a steel. Hot ductility tests were performed afterhomogenisation treatment for 10 h at 1270° C./air of steels in the castand forged condition, respectively. For the forged condition, alsoregeneration treatment at 1050° C./2 h and soft annealing are applied.The holding time at the test temperature was 4 min, except for steel 1and 3 in their cast conditions, and for temperatures equal or higherthan 1200° C. for forged materials. The reason for this is that thesetwo steels were heavily oxidised, which made a correct measuring of thearea contraction impossible. Steel 2, which had a low silicon content,on the other hand, did not give rise to any noteworthy oxidation. Thissteel also had a better hot ductility than steels No. 1 and 3 in thecast as well as in the forged conditions. About 50° C. higher testtemperature could be allowed for steel 2. The results are illustrated inFIG. 5.

Abrasive Wear

The wear resistance was examined via pin-against-disc test with SiO₂ asan abrasive wear agent. Steel 4 had the best wear resistance. The othersteel alloys were equally good.

Discussion

Comparative studies of the examined steels were carried out for theevaluation of the above reported results. Table 5 shows the content ofdissolved carbon, weight-%, and the content of MC-carbides, vol-%, at1050° C., when equilibrium is assumed to apply for the steels 1-3 and5-7, and at 1150° C. for the steels 4 and 8. The values of the aimedcompositions of the steels 5-8 are given as a reference in the table. Itis noticeable that steel 2 has a substantially lower MC-content than theintended content because the vanadium content is lower than according tothe nominal composition of that steel, steel 6 which contained 0.65vol-% MC at TA.

TABLE 5 The content of dissolved carbon, weight-%, and carbon fraction,vol-%, at the indicated austenitising temperature for the examinedalloys 1-4 in comparison with the aimed compositions 5-8 of thesealloys. Steel Optimal T_(A) (° C.) % C at T_(A) % MC at T_(A) 5 1050/30min 0.64 0.89 1 1050/30 min 0.60 0.87 6 1050/30 min 0.65 0.65 2 1050/30min 0.66 0.32 7 1050/30 min 0.65 0.97 3 1050/30 min 0.63 0.95 8 1150/30min 0.37 2.83 4 1150/30 min 0.30 2.71

A comparison of the features of the examined alloys 1-4 is given intable 6. In this table the alloys have been afforded marks varyingbetween 1-4, were 1=lowest and 4=best.

TABLE 6 Comparison of features of examined steel Steel No: 1 2 3 4Hardenability 2 4 3 1 Dimension stability 2 4 3 1 at heat treatmentHardness after high- 4 4 4 4 (however only after hardening temperaturehardening from high temperature Doctilitet/Toughness 2 4 3 1 Wearresistance 2 2 2 4 Fatigue life 4 4 4 2 Pressure strength 4 4 4 4Grindability 4 4 4 2 Machineability 4 3 4 2 Spark machineability 4 4 4 4Polishability 4 4 4 3 Production economy 3 4 4 2

As is apparent from table 6, steel No. 2 has a better combination offeatures than the other examined and evaluated materials. Particularly,it is better as far as the most important product features areconcerned. Possibly, the lower content of MC-carbides is an unfavourableaspect of steel 2, because it might reduce the resistance against graingrowth. It is therefore an experience of the experiments that thevanadium content should be increased from nominally 0.40% to 0.50% inorder to give a wider margin against grain growth during heat treatment.The experiments also indicate that an narrow range exists for thevanadium content for the provision of a desired resistance against graingrowth without causing the carbide content to be too high with referenceto the toughness of the steel and also that the carbon content should beincreased to nominally 0.72% and be maintained within a rather narrowrange about that content for the provision of 60-62 HRC after heattreatment. The contents of P, S, N, and O should be kept at a very lowlevel in order to maximise ductility and toughness. Other carbide- andnitride formers such as Ti, Zr, and Nb should most conveniently berestricted to max. 0.005%. Against this background, a cold work steelaccording to the invention should have the nominal composition given intable 7.

TABLE 7 Nominal composition, weight-%, of a steel according to theinvention, steel No. 9, and amount of dissolved C and amount ofcarbides, vol-%, at 1050° C. MC* C Si Mn P S Cr Mo V N O C* vol-% 0.720.20 0.50 ≦0.010 0.0010 5.0 2.30 0.50 ≦0.010 ≦0.0010 0.67 0.6 Balanceiron and anavoidable impurities *Theorotically calculated at equilibriumaccording to the Thermo-Calc method.

Experiments at a Production Scale

A 65 ton production heat was manufactured in an electric arc furnace,the aimed composition of the heat corresponding to steel No. 9 accordingto table 7. A number of ingots were made of the molten metal, and theingots were forged to the shape of bars having different dimensions,including bars with the dimensions Ø330 mm and Ø254 mm, respectively,steel No. 10 and No. 11 in table 8. In the same table, also the chemicalcomposition of a reference material, steel No. 12, is given. Thatmaterial had the shape of a forged bar with the dimension Ø330 mm. Intable 8, not only phosphorus and sulphur are impurities. Also tungsten,cobalt, titanium, niobium, copper, aluminium, nitrogen, and oxygen inthe given amounts are impurities. Other impurities are not indicated butlie below allowed levels. The balance was iron.

Test rods were taken out from the manufactured bars. FIG. 7 shows themicrostructure of the steel in a sample taken in the centre of the barof steel No. 11. The sample was hardened by austenitising at 1025° C./30min, air cooling and subsequently annealed at 525° C./2×2 h. As isapparent from the figure, the steel had an even microstructureconsisting of tempered martensite without any primary carbides.

The ductility was investigated by impact tests performed on un-notchedtest rods taken from the bars in the most critical positions and themost critical direction, respectively. The test rods of steel No. 10 andNo. 11 were hardened to 61.0 HRC (Rockwell hardness), and 60.5 HRC,respectively, by austenitising at 1025° C./30 min air cooling andtempering at 525° C./2×2 h. The samples of steel No. 12 were hardened to60.2 HRC by austenitising at 1050° C./30 min, air cooling and temperingat 550° C./2×2 h. The absorbed impact energies are shown in the barchart in FIG. 6. In the chart, the denominations CR1 and CR2 areemployed, where CR1 means test rod from round bar, taken in the surfaceof the bar in the longitudinal direction of the bar and with the impactdirection in the square direction of the bar (next most unfavourableconditions), and CR2 means test rod from round bar, taken in the centreof the bar and in other respects according to CR1 (most unfavourableconditions).

As is apparent from the diagram in FIG. 6, a superiorly much betterductility was measured for the steels according to the invention thanfor the reference material when the hardness of the steels of theinvention were equal or even slightly higher than the hardness of thereference material, as a result of comparable impact test withun-notched, hardened and tempered samples of steels manufactured at aproduction scale.

1. A hot worked matrix steel consisting of (in weight-%): C 0.60-0.85 Si0.05-0.5  (Si + Al) ≦0.5 Mn 0.1-2.0 Cr 4.5-5.5 Mo 1.5-2.6 W ≦1.0 (Mo +W/2) 1.5-2.6 V 0.42-0.65 Nb ≦0.1 Ti ≦0.1 Zr ≦0.1 Co ≦2.0 Ni ≦2.0 S≦0.003

optionally, up to 30 ppm B, balance iron and unavoidable impurities,wherein the steel after hardening and tempering at 520-600° C. (2×2 h)has a hardness of 57-63 HRC and an un-notched impact energy in thetransverse direction of 20-100 J, wherein the steel consists of temperedmartensite, wherein the steel contains 1.04 vol. % or less of primaryprecipitated vanadium carbides, and wherein the steel is void of primarycarbides other than primary precipitated vanadium carbides.